Method of preparing current collectors for electrochemical cells

ABSTRACT

A process is recited for plating elongated current collectors with indium. The current collectors have particular utility in alkaline electrochemical cells, particularly zinc/manganese dioxide alkaline cells containing &#34;zero-added&#34; mercury. The process of the invention involves electroplating conductive wire with indium and then drawing the plated wire to a lesser diameter. The drawn plated wire may then be cut to the desired lengths, typically in the shape of a nail, for use as current collectors within the alkaline cell.

The invention relates to alkaline electrochemical cells and a method ofpreparing indium plated current collectors for such cells.

Alkaline electrochemical cells are used to power a variety of devicessuch as flashlights, radios, and other electronic devices. These cellscontain a zinc anode, an alkaline electrolyte, a manganese dioxide(MnO₂) cathode, and an electrolyte permeable separator film, typicallyof cellulose. The anode contacts the negative terminal and the MnO₂cathode contacts the positive terminal. In the past the zinc anode hasbeen amalgamated with mercury. It is desired to produce alkaline cellswith reduced mercury for example "substantially mercury-free" or"zero-added" mercury cells, for environmental concerns. ("Substantiallymercury-free" cells are herein defined as containing less than about 50parts mercury per million parts total cell weight and "zero-added"mercury cells are defined as cells containing no added amounts ofmercury, commonly resulting in less than about 10 parts mercury permillion parts total cell weight.) In practice it has been difficult tofind environmentally compatible substitutes for mercury.

A metal current collector in the shape of a wire or nail is insertedinto the anode material and conducts electric current evenly from thezinc to the anode terminal. The current collector is typically made ofbrass or copper but may also be composed of cadmium, pure zinc or othermetals. A current collector nail is typically made by cold-forming brassor copper wire to the desired shape and then cutting the wire to thedesired length. It is known to coat the nail with other metals, forexample, lead, indium, cadmium and gallium. The coated nail reduces theamount of gassing which may occur in the cell, especially in"substantially mercury-free" or "zero-added" mercury cells. Indium is anattractive coating material for the collectors because it is less of anenvironmental hazard than lead, cadmium, gallium and mercury.

The invention will be better understood with reference to the drawingsin which:

FIG. 1 is a schematic flow diagram depicting the process steps of theinvention.

In the process of the present invention, brass, copper or otherconventional current collector wire is electroplated with indium beforethe wire is cold-formed and cut into individual current collectors,typically in the shape of a nail. The indium plated wire is thencold-formed to form a head portion for each current collector by using apunch and die, followed by cutting the wire to form individualcollectors. After plating the wire, but prior to cold-forming andcutting it into individual current collectors, the wire is drawn-down tothe desired collector diameter. The final collector diameter may varydepending on the size of the cell. The draw-down may typically be to adiameter which is between about 85 and 95 percent of the diameter of theplated wire before drawing. Surprisingly, although the wire has beenplated with indium before draw-down, the drawing does not impair thedegree of surface smoothness or continuity of the indium plate thereon.In fact the draw-down increases uniformity in the plated surface andenhances its smoothness and luster. The draw-down also increases theadhesion of the plating to the wire. These enhanced properties arebelieved to increase the effectiveness of the plated current collectorsin reducing cell gassing and in reducing load voltage instability duringcell discharge.

In the process of the invention, wire 10 (FIG. 1) is passed to acleaning step 1 primarily to remove grease and oil deposits from itssurface. The wire is typically of brass, in its unburnished state, andmay typically have a diameter between about 1.3 and 1.9 mm. Electrolytic(cathodic) cleaning is preferred wherein the wire is the cathode(negative electrode) and stainless steel may be the anode (positiveelectrode). The electrolyte may be an aqueous solution of sodiumhydroxide or other commercially available alkaline cleaner. The wire maybe subjected to cathodic cleaning by passing a direct current betweenthe electrodes at a current density between about 50 and 100 amp. persq. ft., typicaly about 80 amp. per sq. ft. of wire surface for about 2to 10 seconds, while maintaining the bath at a temperature between about45° to 65° C. The cathodic cleaning removes grease, oil and surfaceparticles from the wire.

The cleaned wire 20, may then be passed from step 1 to a second cleaningstep 2 wherein the wire is subjected to anodic electrolytic cleaningwhich removes surface oxides from the wire surface. In this step theelectrolyte bath may be an aqueous solution of sulfuric acid or otherinorganic acid. The wire forms the anode (positive electrode) andstainless steel may be used for the cathode. The wire may be subjectedto electrolytic cleaning in step 2 by passing a direct current betweenthe electrodes at a current density of about 50 amp per sq. ft. of wiresurface for about 3 seconds while maintaining the bath at about ambienttemperature (21° C.).

The anodically cleaned wire 30, may then be passed to a step 3 whereinthe wire surface is activated by acid treatment. In this step the wiremay be treated with sulphamic acid (HSO₃ NH₂) at ambient temperature,typically for about 1 second. This causes the wire surface to becomeslightly roughened or etched to make it more receptive to indiumplating.

The wire 40 from step 3, may then be passed to step 4 wherein indium iselectroplated onto the wire's surface. The plating is effected in step 4by subjecting the wire to electrolysis, wherein the wire forms thecathode (negative electrode) and indium (99.99% pure), forms the anode,in an electrolyte bath of an aqueous solution of indium sulphamate(In(SO₃ NH₂)₃) at a concentration between about 50 and 100 gm of indiummetal/liter. The plating is accomplished in step 4 by passing a directcurrent between the electrodes at a current density of between about 50and 150 amp. per sq. ft., preferably between about 90 and 110 amp. persq. ft. of wire surface. The plating is conducted for a period of time,typically about 3 seconds or somewhat longer, sufficient to produce alayer of indium of between about 0.5 and 10 micron, typically about 1micron thickness on the wire.

The plated wire 50, is then passed to step 5 wherein it is drawn-down toa diameter which is between about 85 and 95% of its original diameter.The draw-down is accomplished by pulling the wire through an aperture ofdesired diameter in a hard material, preferably diamond. The draw-downis carried out under ambient conditions. The drawn wire is cold-formedand cut to form individual current collectors for use in alkaline cells,typically conventional Zn/MnO₂ alkaline cells containing "zero-added"mercury.

The following is a specific example of the process of the invention:

EXAMPLE 1

Brass wire (70 wt.% copper; 30 wt.% zinc) having a diameter of 1.46 mmand in an unburnished state is passed to a first cleaning step (the wiremay be in a burnished or partially burnished state at this point, butsuch condition is rendered unnecessary by the present process) where itis cathodically cleaned as above described using an aqueous solution ofsodium hydroxide as the electrolyte bath. This step serves to removeoil, grease and dirt from the wire's surface.

The wire is then passed to a second cleaning step wherein it isanodically cleaned in the manner above described. An electrolysis bathis used containing 20 vol. % sulfuric acid, to remove remaining surfaceoxides. The cleaned wire is then subjected to a third step involvingsurface activation by acid treatment. This is a non-electrolytic stepwhereupon the wire is passed through a solution of 10 vol. % ofsulphamic acid at ambient temperature for one second. This treatmentserves to prepare the wire surface for indium electroplating by causingthe surface to become slightly roughened or etched.

The wire is then passed to a fourth step wherein indium is electroplatedonto the wire's surface. In the electroplating step the wire forms thecathode (negative electrode). The anode (positive electrode) is formedof indium (99.99 wt. % pure). The electrolyte bath is an aqueoussolution of indium sulphamate at a concentration of 60 gm indiummetal/liter. The electrolyte bath also contains 50 gm of sodiumchloride/liter. The bath temperature is kept below 30° C. The plating iscarried out at a current density of 100 amp/sq. ft. of wire surface. Thedriving voltage is regulated to about 12 volts to produce this desiredcurrent density. The electroplating is carried out for 7 seconds to forma layer of indium of about 1 micron thickness on the brass wire.

In a fifth step, the wire is drawn-down from its original diameter ofabout 1.46 mm to about 1.39 mm by pulling through a diamond orifice of1.39 mm diameter. Although the wire is drawn down to about 5% reductionin diameter, the plated thickness of indium on the wire remainsessentially unchanged at about 1 micron. Surprisingly, the drawingcauses no disruption in the indium surface continuity or integrity. Tothe contrary, the draw-down produces a noticeably higher smoothness andvisibly greater luster of the plated wire surface. It also increasesadhesion of the plating to the wire surface.

The following examples illustrate the advantages derived fromapplication of the indium coated current collectors made by the processof the invention. (All compositions are by weight unless otherwisespecified.)

Comparative Example A

A conventional zinc/manganese dioxide alkaline size AA cell is preparedwith conventional cathode active material, electrolyte and separatormembrane as illustrated in U.S. Pat. No. 4,740,435, wherein the anodeforms the central core of the cell, the cathode is located around theanode with the separator membrane therebetween. The cell has"zero-added" mercury and contains less than 10 parts mercury per millionparts total cell weight. The anode current collector is a brass nailplated with lead. The plated nail has a diameter of 1.39 mm and a lengthof 31 mm. The cathode is an admixture of electrolytic manganese dioxide,graphite and an aqueous solution of KOH. The separator membrane is aconventional electrolyte permeable membrane. The electrolyte is anaqueous solution of KOH containing 40 wt % KOH and a small amount of ZnOconventionally employed in such electrolyte. The anode is a zinc slurrycontaining mercury free zinc alloy powder conventionally used inalkaline cells containing "zero-added" mercury. The zinc powder is a99.9 wt % zinc alloy containing about 1000 ppm indium. The zinc slurryalso contains aqueous KOH solution, acrylic acid copolymer (CARBOPOL) asgelling agent; and a small amount of an organic surfactant (an organicphosphate ester surfactant sold under the trade designation GAFAC RA600,as described in U.S. Pat. No. 4,195,120).

The cell in this example produces a nominal voltage of about 1.5 voltsand is discharged under a 3.9 ohm load. The cell is tested for shockresistance and voltage stability by tapping or jolting the cell over thecell discharge life. A voltage drop usually between about 600 and 800millivolts (average about 700 millivolts) typically occurs upon impact.

The cell in this example evolves 2.5 milliliters of hydrogen at 71° C.over a period of 4 weeks before discharge. (Holding cells at 71° C.(160° F.) for a period of one week may be equivalent to about one yearof shelf-life of such cells at room temperature.) This volume ofhydrogen gas evolution is considered to be unacceptably high.

EXAMPLE 2

The same AA alkaline cell as in Comparative Example A is prepared with"zero-added mercury" and containing less than 10 parts mercury permillion parts total cell weight and is identical in every respect to thecell in Comparative Example A, except that the anode current collectoris an indium plated brass nail made by the above described process ofthe invention. The nail is formed by drawing-down indium plated wirefrom a diameter of 1.46 mm to 1.39 mm and then cold-forming and cuttingit to a length of about 31.5 mm as described in the process of theinvention. The plated indium on the brass nail has a thickness of about1 micron.

The cell in this example produces a nominal voltage of about 1.5 voltsand is discharged under a 3.9 ohm load. The cell is tested for voltagestability by tapping or jolting it over the cell discharge life. Avoltage drop typically between about 40 and 100 millivolts (averageabout 50 millivolts) occurs upon impact which is significantly less thanthat encountered in Comparative Example 2.

The cell in this example evolves about 0.04 milliliters of hydrogen at71° C. over a period of 4 weeks before discharge.

This gassing level is significantly less than that of ComparativeExample 2 and represents a commercially acceptable level.

Although the present invention is described with respect to preferredembodiments, it should be recognized that other embodiments are possiblewithout departing from the concept of the invention. For example, othercleaning methods are possible for cleaning the wire and removing surfaceoxides prior to electroplating. Modifications to the electroplatingsolution may also be possible. Therefore, the present invention is notintended to be limited to the specific embodiments, but rather isdefined by the claims and equivalents thereof.

What is claimed is:
 1. A method of manufacturing a plurality of elongated current collectors for alkaline electrochemical cells having an anode and a cathode comprising the steps of:a) electroplating a metal conductive wire with indium to form an indium plated wire, wherein the indium plating on said conductive wire has a thickness of between about 0.1 and 10 micron; and b) drawing the indium plated wire formed in step a) to reduce the diameter thereof; and c) inserting the drawn indium plated wire into said anode.
 2. The method of claim 1 wherein the drawing in step b) reduces the diameter of the plated wire by between about 5 and 15 percent of the diameter of said plated wire formed in step a).
 3. The method of claim 1 wherein the electroplating in step a) is carried out by immersing said conductive wire in an electrolyte bath comprising an aqueous solution of indium sulphamate, said conductive wire forming the cathode (negative electrode) and indium forming the anode (positive electrode); and passing direct current between said electrodes, whereupon indium plates onto said conductive wire.
 4. The method of claim 3 wherein the electrolyte bath comprises a concentration of indium sulphamate yielding between about 50 and 100 gm/liter of indium metal.
 5. The method of claim 3 wherein the current passed between said electrodes has a current density between about 50 and 150 amp. per sq. ft. of wire surface.
 6. A method according to claim 1 wherein prior to step a) the surface of the conductive wire is cleaned to remove surface dirt and oxide.
 7. A method according to claim 6 wherein said wire is electrolytically cleaned.
 8. A method according to claim 7 wherein the surface of said wire is anodically activated prior to said electroplating.
 9. A method according to claim 1 wherein the thickness of said plated indium is not reduced by said drawing. 